MFI zeolites are used as highly selective catalysts that utilize uniform pores originating from the zeolite structure.
However, since the size of the pores of a typical MFI zeolite is less than 1 nm, the size of molecules that can be reacted by using the MFI zeolite as a catalyst is limited. Therefore, researches have been conducted on MFI zeolites that have mesopores (2 to 50 nm) larger than micropores (less than 2 nm) so that the zeolites can serve as a catalyst for use in reactions of larger molecules (see, for example, Non-Patent Literature 1). The required size of mesopores varies depending on the size of reactant molecules. Conventionally, trials have been conducted on the formation of mainly mesopores of less than 10 nm, and only a small number of reports have been made on the formation of mesopores of 10 nm or larger.
Several methods for producing an MFI zeolite having mesopores have been proposed.
For example, in one disclosed method, mesopores are formed by eluting a silica component by alkali treatment (see, for example, Non-Patent Literature 2). However, the formed mesopores were small, less than 10 nm.
In another disclosed method, fine carbon particles are mixed with a zeolite when the zeolite is crystallized. Then the fine carbon particles are removed by firing to thereby form mesopores (see, for example, Patent Literature 1). With this method, the distribution of the pores was broad.
In another disclosed method, mesopores are formed using a surfactant (see, for example, Patent Literature 2). However, only examples in which mesopores with a pore diameter of 2.9 nm were formed with this method were disclosed. When mesopores are formed using a surfactant, ordered pores with adjacent pores separated by walls are formed. These walls hinder migration of materials and adversely affect a catalytic reaction. In addition, the surfactant is expensive, and also the step of removing the surfactant is necessary. These hinder industrialization of the method.
In still another disclosed method, ordered mesopores are formed using a surfactant (see, for example, Patent Literature 3). Also in this method, as in Patent Literature 2, walls separating adjacent pores and acting as obstacles to migration of materials are present in the obtained zeolite.
In another disclosed method, fine crystals of 6 nm are aggregated to form mesopores between the aggregated crystals (see, for example, Non-Patent Literature 1). However, also with this method, the formed mesopores were small, less than 10 nm. Moreover, Patent Literatures 4 to 6 propose other methods for aggregating fine MFI zeolite crystals so that spaces between the crystals are used as mesopores.